(a) Field of the Invention:
The present invention relates to a servo control system for precisely moving an object, which is to be controlled and will hereinafter be called a "control", to a target position.
(b) Description of the Related Art:
Servo control systems have found utility in many fields such as automatic drafting machines, machine tools and robot-assisted controls. One example of these servo control systems will next be described in brief with reference to drawings.
FIG. 1 is a block diagram of a conventional servo control system, in which numeral 1 indicates a microcomputer for commanding and outputting target distance and velocity of movement for a control. Designated at numeral 2 is a pulse distributor which generates pulses corresponding to the target distance of movement outputted from the microcomputer 1. Here, the frequency of these pulses is converted in accordance with the target velocity commanded. A subtractor is indicated at numeral 3 and serves to output a signal indicative of the difference in phase between the pulses from the pulse distributor 2 and a phase signal corresponding to a current position of the control and outputted from a position detector 9 to be described subsequently. There are also illustrated another subtractor 4 for computing the difference between a phase difference signal from the subtractor 3 and a rotational speed signal from a tachometer generator 8 to be described subsequently, and an amplifier 5 for amplifying a difference signal from the subtractor 4. Designated at numeral 6 is a servomotor which is driven at a rotational speed always kept in conformity with each velocity command in accordance with a signal from the amplifier. Numeral 7 indicates a load connected to the servomotor 6, namely, the control. In the case of an automatic drafting machine for example, the load 7 may be a head which carries a writing instrument and moves on a sheet of recording paper. There is also shown the tachometer generator 8 connected to the servomotor 6. The tachometer generator 8 outputs as a signal a current rotational speed of the servomotor 6. The position detector 9 is connected to the servomotor 6 and outputs data on a current position of the load 7.
Owing to such a construction, the control is caused to move to a target position at a velocity conforming with a value commanded.
In the above-described conventional servo control system, the control of velocity has been performed by generating pulses at the pulse distributor 2 and then converting them into pulses of a frequency conforming with a commanded value of velocity. In other words, the pulse distributor 2 has been constructed to produce outputs at intervals conforming with a given velocity. To obtain such outputs, the pulse distributor 2 must have a broadband conversion function for pulse frequencies, resulting in the problem that the circuit construction of the pulse distributor 2 inevitably becomes extremely complex and large.
With the conventional servo control system, the acceleration changes considerably so that impacts and vibrations are exerted to an associated machine. They in turn cause loosening of screws, separation of bonded parts, etc., whereby the service life and accuracy of the machine are reduced. Moreover, vibrations in the course of movement tend to lead to inconvenience, for example, such that desired line segments cannot be drawn precisely in the case of an automatic drafting machine or desired cutting cannot be conducted in the case of a machine tool. A pulse frequency determination system is required as a countermeasure for the inconvenience, thereby making the servo control system complex and expensive.